feat(fleet): F4 Phase 1 — orchestrator chat connector abstraction + Matrix design (#616)

From the north-star (#613) orchestrator-chat-connector decision: make the
orchestrator's chat channel a pluggable, user-chosen connector (tmux | discord
| matrix peers) and add Matrix (local homeserver) — starting with the small
uniform abstraction so connectors drop in without touching fleet core.

Phase 1 (this PR — design + scaffold):
- src/fleet/connectors/types.ts: OrchestratorConnector interface (send /
  subscribe / health per the Lead's spec) + message/config types; thread-aware
  via optional threadId so Matrix threads + the future first-party Mosaic
  Discord plugin fit without an interface change. DEFAULT_CONNECTOR_KIND=tmux.
- src/fleet/connectors/registry.ts: extensible factory registry;
  resolveConnectorKind defaults tmux (back-compat); createConnector throws
  ConnectorNotImplementedError until Phase 2 registers factories.
- roster.schema.json: optional connector block (tmux|discord|matrix; matrix
  homeserver/user/room). Secrets via env (gateway pattern), never the roster.
- docs/fleet/f4-matrix-connector.md: interface, config, Matrix client-server API
  mapping, Conduit-default local homeserver, phasing, back-compat.

Self-contained connectors/ module — NO fleet.ts changes, so independent of the
in-flight stacked fleet-config PR (#615).

Verified: 7 connector tests green; tsc/eslint/prettier/sanitize clean; schema
valid JSON.

Refs #616, #613

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EsgTQzV5YUGk1JtCLP4B83

docs/TASKS.md

@@ -58,3 +58,7 @@ Active workstream is **W1 — Federation v1**. Workers should:
 ## F3-m3 — mosaic update re-seeds framework + relaunches agents (#609) — feat/f3-m3-update-reseed
 
 - Status: implemented + tested. Closes R13: `mosaic update` now re-seeds the framework (data-safe MOSAIC_SYNC_ONLY) after the CLI install so shipped launcher/runtime changes activate; `--relaunch` restarts rostered agents; `--no-reseed` opts out. Detail: scratchpads/f3-m3-update-reseed.md.
+
+## F4 — Orchestrator chat connector + Matrix (#616) — feat/f4-matrix-connector
+
+- Status: Phase 1 done (abstraction + scaffold). Connector interface (send/subscribe/health) + registry + roster connector schema + design doc; tmux default/back-compat; matrix/discord factories are Phase 2. 7 tests green; no fleet.ts changes (independent of #615). Detail: scratchpads/f4-matrix-connector.md.